Middle Ear

As mentioned in the previous page, the tympanic membrane serves as the boundary between the outer and middle parts of the ear. The tympanic membrane is commonly known as the “eardrum” because it literally acts like a drum due to its vibrations in response to sound pressure waves. It is roughly around 17.5 mm in diameter. It has a translucent quality and is also only a little thicker than onion skin paper (paper used in crafts, confetti, and lanterns).

Wageih Felfela (2017) stated that the anatomical landmarks of the tympanic membrane include the (1) malleus, (2) anterior mallear fold, (3) posterior mallear fold, (4) pars flaccida, (5) projection of the long process of the incus, (6) pars tensa, and (7) annular ligament.

Layers of the Tympanic Membrane

The tympanic membrane is composed of three layers: (1) the inner, mucosal (endodermal) layer that is continuous with the lining of the middle ear; (2) the middle, fibrous layer made up of circular and radial fibers that are responsible for the sturdiness of the tympanic membrane; and (3) the outer, epithelial (ectodermal layer) that is continuous with the lining of the external auditory meatus.

The tympanic membrane has several parts which will be discussed in the succeeding paragraphs.

Pars Tensa and Pars Flaccida

Sanchez Lopez de Nava (2020) stated that the pars tensa is the inferior portion that makes up 75% of the tympanic membrane. It is described as a tightly stretched fibrous structure. The pars tensa is in-charge of translating sound waves into mechanical movement. Its effective area of vibration is 55 square millimeters. On the other hand, the pars flaccida is the superior portion that composes 25% of the tympanic membrane. This is described as a loose and floppy structure. In fact, the term “flaccida” originates from the word “flaccid” which means “soft, not tense, or not rigid.” The pars flaccida does not take an active role in sound wave production, but is considered as necessary for the proper functioning of the pars tensa.

Annula

Meanwhile, the annula refers to the space or depression between the annular ligament and the external auditory meatus.

The eustachian or auditory tube is another part of the middle ear. This is responsible for the connection between the middle ear and nasopharynx or opening towards the nasal cavity. According to Wageih Felfela (2017), the eustachian tube is almost horizontal in infants and children. Also, the eustachian tube is responsible for equalizing the air pressure between the surrounding environment and the middle ear cavity. It also manages the aeration and drainage of the middle ear.

The Eustachian tube is 35-36 millimeters by adulthood and 17-18 millimeters at birth. Moreover, bones surround the first third of the tube which is approximately three to six millimeters in diameter. On the other hand, an incomplete ring of elastic cartilage surrounds the remainder of the tube. It is also important to note that at rest, the cartilage keeps the eustachian tube closed.

Sanchez Lopez de Nava (2020) stated that the “popping” of the ear during the take-off and landing parts of riding an airplane is a manifestation of the equalization of the air pressure in the ears caused by the opening and closing of the Eustachian tube.

References

Alberti, P. (2006). 2 THE ANATOMY AND PHYSIOLOGY OF THE EAR AND HEARING.

Wageih Felfela, G. M. (2017). Ear Anatomy. Global Journal of Otolaryngology, 4(1). https://doi.org/10.19080/gjo.2017.04.555630

Sánchez López de Nava A, Lasrado S. (2021). Physiology, Ear. In: StatPearls [Internet]. Treasure Island ( FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK540992/